Compostable multilayer structures, methods for manufacture and articles prepared therefrom

ABSTRACT

A compostable multilayer film includes a core layer having a first surface and a second surface, a first blocking reducing layer covering the first surface of the core layer, and a second blocking reducing layer covering the second surface of the core layer. The core layer comprises a lactic acid residue-containing polymer having a glass transition temperature (Tg) below 20° C. At least one of the first and second blocking reducing layers comprise a semicrystalline aliphatic polyester. The core layer may be peroxide modified polylactide polymer which exhibits bridging between polylactide polymer chains.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 08/642,329,filed May 3, 1996, and which issued as U.S. Pat. No. 5,849,401 on Dec.15, 1998, which is a continuation-in-part of application Ser. No.08/535,706, filed Sep. 28, 1995, and which issued as U.S. Pat. No.5,849,374 on Dec. 15, 1998, application Ser. Nos. 08/642,329 and08/535,706 being incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to compostable multilayer structures,methods for the manufacture of compostable multilayer structures, andarticles prepared from compostable multilayer structures. Moreparticularly, the compostable multilayer structures are films havingdesirable properties of flexibility and tear resistance and can be usedto provide disposable bags or wrappers.

BACKGROUND OF THE INVENTION

Plastic trash bags and wrappers are primarily made of hydrocarbonpolymers such as polyethylene, polypropylene, or polyvinyl polymers.While hydrocarbon polymers can be useful for commercially manufacturingtrash bags and wrappers having adequate flexibility and puncture andtear resistance, they are resistant to degradation and mineralizationand have a tendency to build up in land fills. Under most conditions,hydrocarbon polymers take a long time to decompose. In addition,hydrocarbon polymers are not manufactured from renewable resources.

Hydrocarbons have been combined with starch in attempts at increasingdegradability. Trash bags which incorporate starch can be physicallydegradable, which means that they become broken into many small parts asthe starch biodegrades. The hydrocarbon component, however, remainsresistant to degradation and mineralization. In certain circumstances,it is believed that the hydrocarbon component has a tendency toencapsulate the starch thereby preventing further biodegradation of thestarch. Furthermore, materials incorporating large amounts of starch canbe very sensitive to moisture and can have mechanical properties whichvary considerably with humidity.

Attempts have been made at developing thermoplastic films havingdegradable properties. For example, U.S. Pat. No. 4,133,784 to Otey etal. describes degradable mulch films with improved moisture resistanceprepared from starch and ethylene/acrylic acid copolymers. U.S. Pat. No.5,091,262 to Knott et al. describes a multilayer polyethylene filmcontaining a starch filled inner layer, and prodegradant filled outerlayers. U.S. Pat. No. 5,108,807 to Tucker describes a multilayerthermoplastic film having a core layer made of polyvinyl alcohol, andouter layers made of polyethylene and prodegradant. U.S. Pat. No.5,391,423 to Wnuk et al. describes multilayer films prepared fromvarious biodegradable polymers for use in disposable absorbent products,such as diapers, incontinent pads, sanitary napkins, and pantyliners.

Many biodegradable polymers have been found to possess the desirablecharacteristics of biodegradability and compostability. At roomtemperature, however, many biodegradable polymers are either too brittleto provide the desired puncture and tear resistance necessary forcommercially acceptable trash bags, or they do not have adequatestability for storage and transport. In addition, many biodegradablepolymers are difficult to process into films using commercialmanufacturing lines.

SUMMARY OF THE INVENTION

Compostable multilayer structures with desired properties of flexibilityand tear resistance are provided by the present invention. Thecompostable multilayer structures are preferably in the form of films,sheets, laminates and the like. The compostable multilayer structurescan be manufactured into disposable consumer products such as bags,wrappers, cups, and the like, which can degrade when subjected tocomposting conditions. Preferably, the multilayer structure is in theform of a film.

The compostable multilayer structures can be provided in various layeredarrangements. A preferred compostable multilayer structure includes acore layer having a first surface and a second surface, a first blockingreducing layer covering the first surface of the core layer, and asecond blocking reducing layer covering the second surface of the corelayer. Preferably, the core layer has a glass transition temperature(T_(g)) below about 20° C., and at least one of the first and secondblocking reducing layers includes a semicrystalline polymer compositionand/or has a glass transition temperature above about 50° C.

Applicants discovered that certain desirable properties of compostablepolymer compositions, such as flexibility, tear resistance, and punctureresistance, can be adjusted by controlling the glass transitiontemperature thereof. For example, for many compostable polymercompositions, such as hydrolyzable polymer compositions, reducing the(T_(g)) provides a layer having increased flexibility, tear resistance,and puncture resistance to commercially acceptable levels for bags andwrappers. In addition, Applicants discovered that certain polymerscompositions can be used to provide blocking reducing layers whenapplied over the compostable polymer compositions having increasedflexibility, tear resistance, and puncture resistance. As used in thecontext of the present invention, blocking occurs when polymercomposition layers fuse or stick together. The extent of blocking isevaluated relative to the degree of fusion between the layers ortackiness of the layers. Many polymer compositions having low glasstransition temperature have been found to possess increased incidence ofblocking. Applicants discovered, however, that resistance to blockingcan be adjusted by controlling the glass transition temperatures. Formany compostable polymer compositions such as certain hydrolyzablepolymer compositions, an increased glass transition temperature tends toreduce blocking. In addition, Applicants additionally discovered thatcontrolling the crystallinity of a polymer composition can providereduced blocking.

The layers of the compostable multilayer structures are preferably madeof materials which are compostable, such as polymer compositions whichinclude, for example, hydrolyzable polymers. Exemplary hydrolyzablepolymers include copolymers and polymer blends of poly(trimethylenecarbonate) and polyesters such as poly(lactic acid), poly(lactide),poly(glycolide), poly(hydroxy butyrate), poly(hydroxybutyrate-co-hydroxy valerate), poly(caprolactone),poly(ethylene-oxylate), poly(1,5-dioxepan 2-one), poly(1,4-dioxepan2-one), poly(p-dioxanone), poly(delta-valerolactone),polyethylene(oxylate), polyethylene(succinate), polybutylene(oxalate),polybutylene(succinate), polypentamethyl(succinate),polyhexamethyl(succinate), polyheptamethyl(succinate),polyoctamethyl(succinate), polyethylene(succinate-co-adipate),polybutylene(succinate-co-adipate), polybutylene(oxylate-co-succinate),polybutylene(oxylate-co-adipate). Aliphatic polyesters which arepreferred because of their ability to hydrolyze to generallybiodegradable units. It should be appreciated that lactic acid residuecontaining polymers such as poly(lactide) and poly(lactic acid) arepreferred hydrolyzable polymers because of their composting andbiodegradable properties. Even more preferred are copolymers preparedfrom lactide or lactic acid and epoxidized multifunctional oil, such assoybean oil or linseed oil.

The polymers which can be used to provide the layers of the multilayerstructure should have a molecular weight which is sufficient to providea polymer composition having film or sheet forming properties. Thismeans that the molecular weight should be sufficiently high so that thepolymer composition can form a sheet or film having integrity, and thatthe molecular weight should not be too high that the polymer compositionis too viscous and has problems forming a sheet or film using commercialfilm or sheet forming equipment. Moreover, it should be appreciated thatthe molecular weights of the polymers used to provide the various layerscan be different, reflecting the desired properties of the individuallayers. For example, the molecular weight of the polymer used to preparethe core layer should be sufficiently high to provide sufficient tearstrength and puncture resistance, and the molecular weight of thepolymers used to prepare the blocking reducing layers should besufficient to provide the desired glass transition temperature.

Practically, it is believed that this generally corresponds withpolymers having a number average molecular weight in the range of about50,000 to about 200,000, and a weight average molecular weight in therange of about 100,000 to about 600,000. To provide sufficientflexibility and puncture and tear resistance, it has been found that thepolymer used to prepare the core layer should have a number averagemolecular weight between about 80,000 and 200,000, more preferablybetween about 90,000 and 175,000, and even more preferably between about100,000 and 150,000. The blocking reducing layers should have a numberaverage molecular weight above about 50,000. As will be discussed inmore detail, the selection of the molecular weight of the polymers usedin the various layers can be adjusted to provide viscosity matchingcharacteristics during, for example, coextrusion.

It is understood that the low glass transition temperature isresponsible for providing the multilayer structure with desiredflexibility and tear resistance. Accordingly, it is desirable to providethe glass transition temperature of the core layer below thetemperatures at which the multilayer structure will be used. It has beenfound that for most conditions of use at room temperature, a T_(g) belowabout 20° C. should be acceptable. At cooler conditions, it is preferredthat the core layer should have a T_(g) below about 5° C., and undermore extreme conditions, a T_(g) below about −10° C. would be preferred.

A preferred technique for reducing the glass transition temperature ofthe core layer is to incorporate therein an effective amount ofplasticizers into the polymer composition which forms the core layer.Generally, this means that the plasticizer can be included to provide aconcentration level of about 10 to 35 percent by weight, and morepreferably a concentration level of about 12 to 30 percent by weight. Itis preferred that the plasticizer is biodegradable, non-toxic,compatible with the resin, and relatively nonvolatile. Plasticizer inthe general classes of alkyl or aliphatic esters, ether, andmulti-functional esters and/or ethers are preferred.

When the core layer has a glass transition temperature below thetemperature of use of the multilayer structure, it has been found thatthe core layer suffers from blocking. It should be appreciated thatblocking occurs when polymer layers fuse together. The extent ofblocking is a function of the degree that the layers fuse together.Layers which are highly blocked will be almost totally fused together.Blocking is a particularly undesirable property for certain articlessuch as bags and wrappers which are commonly stored in a roll or otherarrangement where the layers are in contact.

Applicants have found that blocking can be reduced by the incorporationof blocking reducing layers in the compostable multilayer structure.Preferably, the blocking reducing layers are hydrolyzable polymers suchas lactic acid residue containing polymers having a number averagemolecular weight above 50,000. If desired, the blocking reducing layerscan include anti-blocking agents to reduce blocking. Exemplaryanti-blocking agents include poly(hydroxy butyrate co hydroxy valerate),cellulose acetate, cellulose propionate, cellulose butyrate, celluloseacetate propionate, cellulose acetate butyrate, cellulose propionatebutyrate, and mixtures thereof. The amount of anti-blocking agent shoulddetermined based upon its effect in reducing blocking.

In an alternative embodiment, the blocking reducing layer can include apolymer composition including a semicrystalline polymer which providesreduced blocking. Exemplary semicrystalline polymers includepolyethylene(oxylate), polyethylene(succinate), polybutylene(oxalate),polybutylene(succinate), polypentamethyl(succinate),polyhexamethyl(succinate), polyheptamethyl(succinate),polyoctamethyl(succinate), polyethylene(succinate-co-adipate),polybutylene(succinate-co-adipate), polybutylene(oxylate-co-succinate),polybutylene(oxylate-co-adipate), or mixtures or copolymers thereof. Itis generally preferred that the semicrystalline polymer has acrystallinity of greater than 10 J/g as determined by a differentialscanning calorimeter. More preferably, the semicrystalline polymer has acrystallinity of greater than 30 J/g.

In an alternative embodiment of the present invention, the compostablemultilayer structure can be provided as a two layer structure having acore layer and a blocking reducing layer. It is believed that thiscompostable multilayer structure can be stored in the form of a roll sothat both sides of the core layer are adjacent a blocking reducinglayer.

Methods for manufacturing compostable multilayer structures are providedby the present invention. A preferred method includes the step ofcoextruding layers of compostable polymer compositions to form amultilayer structure. The coextrusion step can be carried out by avariety of techniques. One technique involves combining the separatemelt streams either in the die or by simultaneously casting. Analternative technique involves combining or casting two or more streamsonto a substrate, such as paper or other form of web. The layers of themultilayer structure preferably include a core layer having a firstsurface and a second surface and a T_(g) below about 20° C.; a firstblocking reducing layer covering the first surface of the core layer,and having a T_(g) above about 50° C.; and a second blocking reducinglayer covering the second surface of the core layer, and having a T_(g)above about 50° C.

The structure may be a film formed from an extruded melt by any ofseveral means. The structure may be cast and quenched, either onto adrum, a belt, in water, or the like. The cast film may be subsequentlyoriented, either uniaxially or biaxially, using conventional equipmentsuch as drawing on heated rollers or using a tenter-frame, or acombination thereof. The processing operation may also includecrystallization (of the outer layers) and/or heat-setting of the film.The biaxially oriented film can also be subjected to additional drawingof the film in the machine direction, in a process known as tensilizing.

The film may also be processed in a blown-film apparatus, in order toachieve direct biaxial orientation directly from the melt or in adouble-bubble process. The blown-film process is known in the art, as isthe double-bubble process. In the blown film process the annular tube isinflated as it leaves the extruder and is cooled with an air ring, priorto collapsing and winding. The double-bubble process first quenches thetube, it is then reheated and oriented by inflating at a temperatureabove the T_(g) but below the crystalline melting point (if the polymeris crystalline).

For lactic acid residue containing polymers based outer layers, thepreferred temperature for orienting in the double bubble process isbelieved above the T_(g) but below 90° C., and preferably below 75° C.If orientation occurs at a temperature which is too high, it may bedifficult to obtain sufficient crystallinity, and the resulting polymercomposition will not be resistant to blocking at temperatures greaterthan 50° C.

The film may also be subjected to rolling, calendaring, coating,embossing, printing, or any of the other typical finishing operationsknown in the art.

The core layer, with a T_(g) less than 20° C. and (typically) amorphous,can be difficult to handle in pelletized form. A preferred method forpreparing the core layer is to start with a polymer which has a T_(g)greater than about 40° C. and inject the plasticizer into the extruderduring the film forming process. This “on-line” plasticization isadvantageous because it removes the need to pelletize and handle the lowT_(g) material.

Compostable multilayer sheets are provided by the present invention. Thesheet has a thickness greater then 10 mils (0.010 inch). The sheet canbe used as thermoformed rigid container, cups, tubs, dinnerware, etc. Inmost applications, it is understood that the sheet will have a thicknessless than 150 mils.

A compostable film is provided by the present invention, wherein thecompostable film includes a lactic acid residue containing polymer, andhas a tear resistance of greater than 50 gm_(f)/mil at 23° C. accordingto ASTM D1922-89, and exhibiting substantially no blocking when foldedback on itself and held together under a pressure of 180 g/in² at 50° C.for two hours, and preferably for 24 hours. Preferably, the film has atear resistance of at least 65 gm_(f)/mil, and more preferably at least80 gm_(f)/mil at 23° C. according to ASTM D1922-89.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a multilayer structure in the formof a film according to the principles of the present invention;

FIG. 2 is a cross-sectional view of an alternative embodiment of amultilayer structure in the form of a laminate having a paper substrateaccording to the principles of the present invention; and

FIG. 3 is a graph comparing the rate of biodegradation of the multilayerfilm of Example 2, kraft paper, and cellulose.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a multilayer structure havingcompostable properties. This means that the multilayer structure willbreak down and become part of a compost upon being subjected tophysical, chemical, thermal, and/or biological degradation in a solidwaste composting or biogasification facility. As used in thisapplication, a composting or biogasification facility has a specificenvironment which induces rapid or accelerated degradation. Generally,conditions which provide rapid or accelerated degradation, compared withstorage or use conditions, are referred to herein as compostingconditions. In the context of the present invention, the multilayerstructure may be referred to as a compostable multilayer structure.

In order to provide a compostable multilayer structure, the componentsof the multilayer structure should be compostable and biodegradableduring is composting/biogasification, or in compost amended soil, at arate and/or extent comparable to that of known reference materials suchas cellulose or paper. Basically, this means that the components shouldbe degradable within a time frame in which products made therefrom,after use, can be recycled by composting and used as compost. It shouldbe understood that certain materials such as hydrocarbons and otherpolymeric resins including polyethylenes, polypropylenes, polyvinyls,polystyrenes, polyvinyl chloride resins, urea formaldehyde resins,polyethylene terephthalate resins, polybutylene terephthalate resins,and the like are not considered compostable or biodegradable forpurposes of this invention because they take too long to degrade whenleft alone in a composting environment.

It is preferred that as large a percentage as possible of the materialsmaking up the multilayer structure should be compostable andbiodegradable. Preferably, the materials can be chemically orbiologically broken down, then mineralized by microorganisms in abiologically active environment to simple molecules, such as, carbondioxide, methane, and water, leaving biomass and naturally occurringassimilation products. In assessing biodegradability, both the rate ofbiodegradation and the ultimate extent of biodegradation are importantconsiderations. It is preferred that the rate of biodegradation and theextent of biodegradation of the materials used in the multilayerstructure should be comparable to known reference materials such ascellulose or paper.

The rate and extent of biodegradation of the multilayer structure can becorrelated to known biodegradable materials, such as kraft paper orcellulose, using ASTM D5338-92 Test Method for Determining AerobicBiodegradation of Plastic Materials Under Controlled CompostingConditions. This is a laboratory test which compares the rate ofbiodegradation of a test sample to that of a known biodegradablematerial by determining the amount of CO₂ evolved from the compost withand without the test sample. A modified version of the ASTM D5338-92test can be used to more conveniently approximate large scale compostingconditions. This modified test is referred to as the first modified testand is performed according to ASTM D5338-92 except that a constanttemperature of 58° C. is provided. The amount of material biodegraded iscalculated based upon measuring the amount of carbon dioxide evolvedtherefrom. A second modified version of ASTM D5338-92 can be used todetermine the degradation at soil conditions. The second modified testis carried out according to ASTM D5338-92 except that a temperature of30° C. is used and the media is soil at approximately 70% of itsmoisture holding content (ASTM D 425).

Test results for biodegradation according to the first modified ASTMD5338-92 test are provided in FIG. 3 where the cumulative percentbiodegradation (referred to as the biodegradation value) is measured asa function of time for the multilayer structure prepared in Example 2,kraft paper, and cellulose. The details of this test are described inExample 9. For the results plotted in FIG. 3, a biodegradation value of70 percent at 40 days means that at least 70 percent of the carbon inthe multilayer structure has been converted to carbon dioxide andmicrobial biomass after composting under conditions of the firstmodified test for 40 days. For most multilayer structures of the presentinvention, it is preferred that they have a biodegradation value of atleast 50 percent after 40 days, and even more preferably at least 60percent after 40 days. In addition, it is preferred that they possess abiodegradation value of at least about 70 percent after 60 days. Forslower materials, biodegradation values of at least about 20 percentafter 40 days and/or at least about 30 percent after 60 days can beprovided.

Another way of characterizing the rate and extent of biodegradation ofthe multilayer structure of the invention is to compare it with the rateand extent of biodegradation of known compostable and biodegradablematerials such as kraft paper and cellulose. Generally, it is desirablethat the multilayer structure will have a biodegradation value which isat least about 50 percent, and more preferably at least about 60percent, of the biodegradation value of kraft paper or cellulose after40 days in a standard compost as provided in ASTM D5338-92.

In a preferred embodiment of the invention, the entire multilayerstructure will eventually decompose under composting conditions. Itshould be appreciated, however, that practical composting conditions mayterminate before the active stage of composting is complete. As aresult, multilayer structures of the invention can then be placed insoil or otherwise disposed of before complete decomposition occurs. Itis believed degradation of a multilayer structure which is notcompletely degraded after composting will continue at a slower rate whenplaced into soil.

It should be understood that the multilayer structure of the inventioncan include materials which are not compostable or biodegradable undershort period composting conditions. Such materials can be incorporatedto provide desired physical properties, such as barrier properties, andshould be kept to a minimum since it is desirable to provide amultilayer structure having a high degree of degradability undercomposting conditions. Exemplary types of materials includethermoplastic resins, such as, hydrocarbons, polyesters, polyvinylalcohols, poly(acrylonitrile) polymers, and select highly substitutedcellulose esters. Exemplary hydrocarbons include polyethylene,polypropylene. Exemplary polyesters include aromatic polyesters, suchas, polyethylene terephthalate (PET) and polybutylene terephthalate(PBT).

According to Example 9, the multilayer structure of Example 2 has abiodegradation value of about 90 percent after 60 days according to themodified ASTM D5338 test, conducted at a constant temperature of 58° C.It should be appreciated that the extent of crystallization in thelactic acid residue containing polymers can alter this value. It isbelieved that a more crystalline lactic acid residue containing polymerwill generally biodegrade more slowly than a less crystalline lacticacid residue containing polymer.

The multilayer structure of the invention can be provided as films,sheets, laminates, and the like. Films can be used in applications suchas disposable bags, wrappers, personal hygiene products, packagingmaterials, agricultural mulch films, and the like. Exemplary disposablebags include trash bags, sandwich or snack bags, grocery bags, waste binliners, compost bags, food packaging bags and the like. Exemplarydisposable wrappers include food wrappers such as fast food wrappers,food packaging films, blister pack wrappers, skin packaging and thelike. Sheets can be used in applications including thermoformed rigidcontainers, cups, tubes, dinnerware, cup lids, deli trays and the like.Laminates include coated paper which can be used, for example, as boxes,multiwall bags, multiwall containers, spiral wound tubes (e.g., mailingtubes), and the like. In situations where the multilayer structure is inthe form of a film or sheet, it may be desirable to ensure that the filmor sheet possess the properties of tear resistance, quietness, andimpact resistance. Transparent structures may also be of benefit for anypackaging application, where it is desirable to see the packagecontents. The multilayer structure of the invention can also be preparedusing coextrusion blow molding, to directly produce articles such asrigid containers, tube, bottles, and the like. For certain applications,such as, use in a compostable lawn refuse bag, it may be desirable tohave the multilayer structure substantially transparent to visiblelight. This allows rapid determination of the contents before shippingto a compost facility, or identification of contents under a wrapper.

Applicants found that presently available biodegradable polymers do notgenerally possess desirable physical properties for use as single layerfilms or sheets because they have high glass transition temperatures,poor tear resistance structures which do not rapidly crystallize (if ithas a low Tg), low melting point, or are difficult to process onconventional machines. These particular problems are often encounteredwhen trying to process biodegradable polymers in conventional processequipment.

It is appreciated that certain hydrocarbons can be very useful forforming single layer bags because they have very low glass transitiontemperatures, crystallize quickly, have relatively high meltingtemperatures, and are easy to process. Polyethylene, for example, has aT_(g) of −100° C. and a T_(m) of over 100° C. which makes it ideallysuited for use in producing single layer trash bags.

The glass transition temperature of a polymer is considered too high foruse as a single layer flexible film or flexible sheet if it is above thetemperature at which the polymer will be used for a given application,which can be referred to herein as the ambient temperature (T_(a)). Whenthe polymer is used at a T_(a) which is lower than the T_(g) of thepolymer, the polymer will typically be too brittle which can result incracking or fracturing of a layer formed therefrom. An example of apolymer having a T_(g) which is too high for forming a single layerflexible film is linear polylactic acid polymer which has a T_(g) ofabout 54° C.

It is understood that crystallinity is an important characteristic of apolymer and can be relied upon to reduce blocking. As discussed above,blocking occurs when films or other structures fuse together. It is aparticularly undesirable property when it is exhibited by trash bagsbecause it causes the sides of the bag to stick together, therebypreventing the bag from opening. It is believed that blocking is afunction of the rate and extent of crystallization of a polymer. Forexample, it is understood that if the polymer crystallizes sufficientlyquickly, it is believed that the tendency to block can be reduced. Onthe other hand, polymers which crystallize slowly will have a tendencyto block in the process equipment when recently formed films or sheetsare brought together causing them to fuse. An example of a polymer whichdoes not crystallize sufficiently quickly under processing conditions ispoly(caprolactone). It is believed, however, that for many polymers suchas poly(caprolactone), processing conditions can be modified to reduceblocking. For example, it is believed that the double bubble blown filmprocess can reduce blocking in poly(caprolactone) polymer compositions.

Some biodegradable polymers are not suitable for single layer bagsbecause they have a melting point (T_(m)) which is too low. A low T_(m)renders a polymer difficult to process, and requires cooling below itsT_(m) to induce crystallization. Several aliphatic polyesters have aT_(m) which is too low. Also, if the storage or use temperature exceedsT_(m) then the film will tend to fuse and lose integrity. An exemplaryaliphatic polyester, such as polycaprolactone, requires acrystallization temperatures of room temperature or below which isdifficult to achieve in most blown film or cast film facilities.Exemplary aliphatic polyesters having desirable T_(m), but T_(g) whichis too high, include polyglycolide, polylactide, and poly(hydroxybutyrate).

Applicants have found ways to provide biodegradable polymer compositionshaving glass transition temperatures lower than ambient temperature.Various methods within the scope of the invention include providingblends of polymers or other additives, using copolymers, incorporating aplasticizer, and the like. These methods are discussed in more detailbelow. Although the resulting biodegradable polymer compositions have aglass transition temperature lower than ambient temperature, it has beenobserved that they can suffer from blocking when formed into a film orsheet. In order to overcome the blocking problem, Applicants discoveredthat certain biodegradable polymer compositions, such as, amorphouspolymer compositions having a high T_(g) or semi-crystalline polymercompositions, can be formed into thin layers and used as blockingreducing layers.

The Compostable Multilayer Structure

Now referring to FIG. 1, a preferred embodiment of the multilayerstructure according to the present invention is shown at referencenumeral 10 in the form of a film. The multilayer film 10 includes a corelayer 12, a first blocking reducing layer 14, and a second blockingreducing layer 16. The first blocking reducing layer 14 covers the firstsurface 13 of the core layer 12, and the second blocking reducing layer16 covers the second surface of the core layer 12. In the arrangementshown in FIG. 1, the core layer 12 is in contact with both the firstblocking reducing layer 14 and the second blocking reducing layer 16. Itshould be understood, however, that for one layer to “cover” anotherlayer, it is not necessary that the layers be in contact with eachother. It should be appreciated that another layer or material can beplaced therebetween. For example, a layer of adhesive, polymer, foil, orother material, such as paper, can be placed between the core layer andthe blocking reducing layer. Various properties, such as, vaporresistance, chemical resistance, adhesion, tensile strength, and thelike, can be provided by selecting layers in addition to those shown inthe multilayer film 10.

It should be appreciated that the multilayer film can be providedwithout a second blocking reducing layer. The film could be stored inroll form so that the core layer contacts both sides of the blockingreducing layer. The film could then be unrolled prior to use, forexample, as a wrapper or covering.

In an alternative embodiment of the invention shown in FIG. 2, alaminated paper product 20 is provided. The laminated paper product hasa paper substrate 21, a core layer 22, and a blocking reducing layer 24covering the core layer 22. It should be appreciated in this embodimentthat the paper substrate 21 can also function as a blocking reducinglayer.

The relative thickness of the core layer and the blocking reducinglayers can be determined by taking a cross section and examining byoptical microscopy. A second method would be to take a cross section andexamine with an FTIR probe to determine composition across the profile.A third way is by measuring the flow rate of polymer streams into themultilayer die and calculating the resulting thicknesses. It isgenerally desirable for the blocking reducing layers to be as thin aspossible to provide sufficient resistance to blocking and sufficientcoverage over the core layer. In most structures, including films,sheets and laminates it is believed that the blocking reducing layerswill be considered films, and may herein be referred to as films in thecontext of the present invention. It is understood that the core layer,generally, is primarily responsible for providing flexibility and tearand puncture resistance. Accordingly, it is usually preferred tomaximize the core layer relative to the blocking reducing layers.

It should be appreciated that it would be desirable to recycle or usescrap or regrind recovered during the production of a multilayerstructure in another multilayer structure. In some applications of themultilayer structure, such as thermoform operations, it is common forfairly large amounts of scrap or regrind to be generated. It is oftenpreferred that the scrap or regrind can be remelted and used in theproduction of the core layer of another multilayer structure in order tokeep the economics favorable. Accordingly, maintaining the blockingreducing layers as a small percentage of the overall composition willallow the scrap or regrind to be added back into the core layercomposition with only minor effects on glass transition temperature.

For many applications where the blocking reducing layer is extruded, thethickness of the blocking reducing layer should be sufficient to providea continuous layer and/or desired blocking resistance. It is believedthat this usually corresponds with a lower limit of at least about 0.05mil. If the thickness of the blocking reducing layer is much less than0.05 mil, it has been found to be difficult to maintain a continuouscoating. In most applications, it is believed that the thickness of theblocking reducing layers should be less than 0.5 mil, more preferablyless than 0.3 mil, and even more preferably less than 0.1 mil. The corelayer can be essentially any size so long as it provides the desiredproperties.

For most multilayer films, such as the one depicted in FIG. 1, it isbelieved that the total thickness of the film will usually be less thanabout 10 mil, and more preferably between about 1 mil and about 3 mils.Since it is desirable to keep the ratio of thicknesses of a blockingreducing layer to the overall thickness of the film as low as possible,the percentage of the blocking reducing layer to the overall thicknessshould be between about 5% and 25%. The percentage of the combinedthickness of the blocking reducing layers to the overall thickness ofthe multilayer structure should be less than about 40%, and morepreferably less than about 30%. Accordingly, at least about 60% of thethickness of the multilayer structure should be core layer, morepreferably at least 70%.

It is believed that a multilayer sheet will have a thickness of at leastabout 10 mil or greater. Multilayer layer structures which include apaper layer or substrate can have a thickness, exclusive of papersubstrate, of 0.5-3 mil. Generally, the thickness of the blockingreducing layers will have essentially the same values described above solong as they provide sufficient blocking resistance.

The Materials of the Multilayer Structure

The polymeric material which can be used in the multilayer structure ispreferably a degradable polymer. A preferred type of degradable polymeris hydrolyzable polymers which can be characterized as beinghydrolytically degradable. This means that chemical bonds in themolecule are subject to hydrolysis, thus producing smaller molecules.More preferably, the “hydrolyzable polymers” should additionally be“biodegradable” in a biologically active environment to simplemolecules. It is preferred that the hydrolyzable polymers can bedegraded by water at neutral pH, with cleavage of polymer linkages.Lactic acid residue containing polymers such as poly(lactic acid)polymers and polylactide polymers are preferred hydrolyzable polymersbecause they generally hydrolyze to lactic acid. The hydrolysis oflactic acid residue containing polymers can be manifested by a decreasein molecular weight as the molecules break down, and by a “weight loss”as water soluble lactic acid is formed.

The hydrolyzable properties of polymers can be evaluated by placing asample of the polymer in an aqueous bath buffered to a pH of 7.4 with aphosphate buffer and maintained at a temperature of 75° C. For thepurposes of this application, if the polymer is hydrolyzable, it willshow significant drop in molecular weight within 3 days, and some weightloss within 14 days. For polymers which are water insoluble, the percentof the polymer considered hydrolyzable is the percent of initiallyinsoluble material which is lost from the sample in 50 days. By thisdefinition, many lactic acid residue containing polymers are 100%hydrolyzable. The crystallinity of a polymer, however, can effect itshydrolyzable properties. More crystalline polymers generally take longerto hydrolyze.

The hydrolyzability test for samples containing water soluble polymerswould be modified to recover the solubilized material and determine itsmolecular weight. The fraction of material with molecular weight equalto or less than the expected monomeric degradation product would beconsidered to have been hydrolyzed.

Exemplary types of hydrolyzable polymers include poly(trimethylenecarbonate) and polyesters such as poly(lactide), poly(lactic acid),poly(glycolide), poly(hydroxy butyrate), poly(hydroxybutyrate-co-hydroxy valerate), poly(caprolactone), poly(1,5-dioxepan2-one), poly(1,4-dioxepan 2-one), poly(p-dioxanone),poly(delta-valerolactone), and other polyesters such as those containingresidues of C₂-C₁₀ diols, and terephthalic acid, and the like. Thepolymers can be copolymers and polymer blends of the above polymers.Preferred polyesters are generally aliphatic polyesters which hydrolyzeto biodegradable units.

Lactic acid residue containing polymers are particularly preferred foruse in the present invention due to their hydrolyzable and biodegradablenature. One theory of the degradation of lactic acid residue containingpolymers is that they can be degraded by hydrolysis at hydrolyzablegroups to lactic acid molecules which are subject to enzymaticdecomposition by a wide variety of microorganisms. It should beappreciated, however, that the precise mechanism of degradation is not acritical feature of the present invention. Rather, it is sufficient thatone recognizes that polymers which provide similarly rapid degradationto naturally occurring end products can be useful in the presentinvention. U.S. Pat. No. 5,142,023 issued to Gruber et al. on Aug. 25,1992, the disclosure of which is hereby incorporated by reference,discloses, generally, a continuous process for the manufacture oflactide polymers from lactic acid. Related processes for generatingpurified lactide and creating polymers therefrom are disclosed in U.S.Pat. Nos. 5,247,058; 5,247,059; and 5,274,073 issued to Gruber et al.,the disclosures of which are hereby incorporated by reference. It shouldbe appreciated that selected polymers from these patents having thephysical properties suitable for use in the present invention can beutilized. Generally, polymers according to U.S. Pat. No. 5,338,822issued to Gruber et al. on Aug. 16, 1994 and U.S. patent applicationSer. No. 08/279,732, filed on Jul. 27, 1994, which are incorporated byreference, can be used in the present invention. Exemplary lactic acidresidue containing polymers which can be used are described in U.S. Pat.Nos. 5,142,023; 5,274,059; 5,274,073; 5,258,488; 5,357,035; 5,338,822;and 5,359,026, to Gruber et al., and U.S patent application Ser. Nos.08/110,424; 08/110,394; and 08/279,732, the disclosures of which areincorporated herein by reference. Polylactide polymers which can be usedin the invention are available under the tradename EcoPLA™.

By now it should be appreciated that the term lactic acid residuecontaining polymer includes polymers containing about 50%, by weight, ormore lactic acid residue units which, under certain conditions, willhydrolyze to lactic acid or derivative thereof. The remaining componentsof the lactic acid residue containing polymers can include non-lacticacid residues. Preferably, the lactic acid residue containing polymer isleast about 70%, and more preferably at least about 90%, lactic acidresidue. In a preferred embodiment, the lactic acid residue containingpolymer contains less than about 2%, by weight, non-lactic acid residue.

Lactic acid residue containing polymers are generally prepared frommonomers which include lactic acid, lactide, or combination thereof. Itshould be understood that other structural units which, whenpolymerized, have a structure similar to polymerized lactic acid orlactide can be used. Rather than focusing on how the lactic acid residuecontaining polymers are prepared, it should be understood that what isimportant is that the lactic acid residue containing polymers havecharacteristics which render them susceptible to hydrolysis and therebyenhance degradability or biodegradability. It is these characteristicswhich are important rather than the strict chemical composition of thepolymer. However, polymers which are considered lactic acid residuecontaining polymers include poly(lactide) polymers, poly(lactic acid)polymers, and copolymers such as random and/or block copolymers oflactide and/or lactic acid. Lactic acid components which can be used toform the lactic acid residue containing polymers include L-lactic acidand D-lactic acid. Lactide components which can be used to form thelactic acid residue containing polymers include L-lactide, D-lactide,and meso-lactide.

A particularly preferred type of polylactide polymer includes viscositymodified polylactide which is described in detail in U.S. Pat. No.5,359,026 and U.S. patent application Ser. No. 08/279,732, filed byGruber et al. on Jul. 27, 1994, U.S. Pat. No. 5,594,095, entitled“Viscosity-Modified Lactide Polymer Composition And Process ForManufacture Thereof,” the application being incorporated by reference.Viscosity modified polylactide polymers are important because theyprovide desirable processing characteristics such as reduced viscosity,increased melt strength, and hence improved bubble stability.

The viscosity modified polylactide polymers which can be used in thepresent invention have increased molecular interaction among the polymerchains. The increased molecular interaction being provided by, forexample, providing bridging in the polylactide polymer, providingbranching with the polylactide polymer, and increasing the weightaverage molecular weight of the polylactide. The “bridging” refers toproviding bonding between long polymer polylactide-based chains. Thiscan be accomplished by using free radical generating peroxides to cleavesubstituents from the polylactide backbones thereby generating polymerradicals that can bond with other polymer radicals, or by reaction ofmultifunctional chain extenders to link polymer chains together. The“branching” refers to providing pendent groups from linearpolylactide-based polymer chains or providing long polymer segmentsjoined to one another through a residue. This can be accomplished byintroducing an initiator into the lactide reactants, by using anon-initiating agent such as an epoxidized hydrocarbon or an epoxidizedoil, or by copolymerizing with molecules containing at least two cyclicester rings.

Particularly preferred viscosity modified polylactide polymers includecopolymers of lactide and epoxidized multifunctional oil such asepoxidized linseed oil and epoxidized soybean oil. In many situations,it is preferred that the polymer is prepared from 0.1 to 0.5 weightpercent epoxidized multifunctional oil and molten lactide monomer.Catalyst can be added, and the mixture can be polymerized between about160° C. and 200° C. The resulting polymer preferably has a numberaverage molecular weight of about 80,000 to about 140,000.

It should be appreciated that lactic acid residue containing polymersare sensitive to high temperatures which creates processing and storagelife problems if not adequately addressed. U.S. Pat. No. 5,338,822describes how lactic acid residue containing polymer stability can beprovided during melt processing.

As discussed above, many biodegradable polymers such as non-plasticizedpolylactic acid polymers are generally too brittle for use as singlelayer flexible films and/or sheets. Their T_(g) is generally above 50°C., and it has been observed that they provide a film or sheet havinglow impact resistance and tear resistance. Tear resistance of a typicalpolylactide film having a T_(g) above 50° C. is less than about 6gm_(f)/mil. Other biodegradable polymers, including certain aliphaticpolyesters, exhibit poor tear strength. These physical properties renderfilms or sheets prepared therefrom poor candidates for use as bags orwrappers. Articles such as trash bags, grocery bags, food wrappings, andthe like should be flexible and resistant to tearing and puncturing.

Applicants discovered that by lowering the glass transition temperature(T_(g)) of biodegradable polymers to about 20° C. or less, it ispossible to provide a film or sheet having improved flexibility and tearand puncture resistance. More preferably, it is desirable to lower theT_(g) to below about 5° C., and more preferably below about minus 10° C.These glass transition temperature should be below the temperature atwhich the polymer is used. When the biodegradable polymer is a lacticacid residue containing polymer, a preferred method for lowering theglass transition temperature (T_(g)) is by adding plasticizer thereto.As demonstrated in Example 1, plasticizer can be added to a polylactidepolymer to lower the glass transition temperature (T_(g)) from 60° C.,without plasticizer, to 19° C. at a level of 20 percent, by weight,plasticizer.

For most lactic acid residue containing polymers, it is believed thatthe glass transition temperature can be lowered to desirable levels byadding a plasticizer component to provide a concentration of about 1 to40 percent by weight plasticizer, based on the weight of the polymercomposition. Generally, a sufficient amount of plasticizer should beincorporated to provide a desired reduction in T_(g) and increaseflexibility and tear strength. It is believed that the plasticizer levelshould be above at least 8 percent by weight, and more preferably aboveat least 10 percent by weight, to provide sufficient flexibility andtear resistance. The upper limit on plasticizer can be controlled byother considerations such as loss of film or sheet integrity if too muchplasticizer is used. Furthermore, too high a concentration ofplasticizer will promote migration of plasticizer into the outer layer.Accordingly, the plasticizer should be included to provide aconcentration level of about 10 to 35 percent by weight, preferably aconcentration level of about 12 to 30 percent by weight, and morepreferably a concentration level of about 20 to 35 percent by weight.

The selection of the plasticizer can involve consideration of severalcriteria. Since it is generally desirable to provide as muchbiodegradability as possible, it is preferred to use a plasticizer whichis biodegradable, non-toxic, compatible with the resin, and relativelynonvolatile. Plasticizer in the general classes of alkyl or aliphaticesters, ether, and multi-functional esters and/or ethers are preferred.These include alkyl phosphate esters, dialkylether diesters,tricarboxylic esters, epoxidized oils and esters, polyesters, polyglycoldiesters, alkyl alkylether diesters, aliphatic diesters, alkylethermonoesters, citrate esters, dicarboxylic esters, vegetable oils andtheir derivatives, and esters of glycerine. Preferred plasticizer aretricarboxylic esters, citrate esters, esters of glycerine anddicarboxylic esters. More preferably, citrate esters are preferred sinceit is believed that these esters are biodegradable. These plasticizercan be obtained under the names Citroflex A-4®, Citroflex A-2®,Citroflex C-4®, Citroflex C-4® (from Morflex).

It should be appreciated that plasticizer containing aromaticfunctionality or halogens are less preferred because of their possiblenegative impact on the environment. For example, appropriate non-toxiccharacter is exhibited by triethyl citrate, acetyltriethyl citrate,tri-n-butyl citrate, acetyltri-n-butyl citrate, acetyltri-n-hexylcitrate, n-butyltri-n-hexyl citrate and acetyltriethyl citrate,tri-n-butyl citrate, diisobutyl adipate, diethylene glycol dibenzoate,and dipropylene glycol dibenzoate. Appropriate compatibility isexhibited by acetyltri-n-butyl citrate, acetyltriethyl citrate,tri-n-butyl citrate, diisobutyl adipate, diethylene glycol dibenzoate,and dipropylene glycol dibenzoate. Other compatible plasticizers includeany plasticizer or combination of plasticizer which can be blended withlactic acid residue containing polymer and are either miscible therewithor which form a mechanically stable blend.

Volatility is determined by the vapor pressure of the plasticizer. Anappropriate plasticizer should be sufficiently non-volatile such thatthe plasticizer stays substantially in the composition throughout theprocess needed to produce the multilayer structure, and to providedesired properties when the structure is used. Excessive volatility canlead to fouling of process equipment, and can result in undesiredplasticizer migration. Preferred plasticizer should have a vaporpressure of less than about 10 mm Hg at 170° C., and more preferredplasticizer should have a vapor pressure of less than 10 mm Hg at 200°C.

Internal plasticizer, which are bonded to the lactic acid residuecontaining polymer, may also be useful in the present invention.Exemplary plasticizer which can be bonded to the polymer includeepoxides.

Although plasticized lactic acid residue containing polymers can providedesired tear strength, they have shown severe blocking which makes themunsuitable as single layer bags or wrappers. It should be understoodthat “blocking” is meant to describe the tendency of one layer of astructure to entangle, enmesh or stick to another layer. Thus, twolayers exhibiting high blocking are not easily separated to form, forexample, a bag. Applicants have found that while reducing the T_(g) oflactic acid residue containing polymers enhances flexibility and tearstrength, it also increases or promotes blocking. This feature isdemonstrated by the data in Table 1 in Example 1.

Applicants have tried several methods to reduce the blocking of a singlelayer plasticized lactic acid residue containing polymer film. Onemethod which reduced the blocking tendency was to crystallize the lacticacid residue containing polymer film. Unfortunately, the crystallizedfilm lost desirable properties, such as tear resistance, making thismethod unsatisfactory. Applicants also tried compounding the polymerwith conventional organic or inorganic fillers, such as diatomaceousearth. When used with a plasticized polylactide composition, thistechnique produced results which initially appeared favorable andproduced a film with good bubble stability and no initial blocking.However, after a short storage period, the films began to fuse or block.

Applicants found that multilayer structures could be created which wererelatively resistant to blocking over time and which retained thedesirable properties of a plasticized lactic acid residue containingpolymer composition, such as, elongation and tear resistance. Theblocking was reduced by incorporating blocking reducing layers whichcover the core layer of plasticized lactic acid residue containingpolymer. The blocking reducing layers could have a variety ofcompositions, provided that they reduce blocking.

The Blocking Reducing Layer

Five preferred types of compositions for forming the blocking reducinglayers are described below. A preferred first composition for preparingthe blocking reducing layer includes amorphous lactic acid residuecontaining polymer having a T_(g) above 50° C. It is believed that thehigh glass transition temperature of the amorphous lactic acid residuecontaining polymer is responsible for reducing or preventing blocking.Thus, blocking can be reduced provided that the ambient or usetemperature is below the T_(g) of the blocking reducing layer. It isbelieved that at temperatures below the T_(g) of the polymer, themolecules in the polymer are not sufficiently mobile to cause blocking.

A preferred second composition which can be used for preparing theblocking reducing layer includes semicrystalline lactic acid residuecontaining polymer. A semicrystalline lactic acid residue containingpolymer will generally have an optical purity of greater than 85% eitherR or S lactic acid residues, although the overall composition can beless optically pure if the polymer is a block copolymer, rather thanrandom. The semicrystalline lactic acid residue containing polymerprovides blocking resistance to higher temperatures than the amorphouslactic acid residue containing polymer, with no blocking observed evenat temperature of 90° C.

A preferred third composition which can be used for preparing theblocking reducing layer includes lactic acid residue containing polymerand a high glass transition temperature polymeric additive for reducingblocking. Preferred high T_(g) polymeric additives include polymers witha T_(g) greater than about 50° C., and more preferably greater thanabout 90° C. The most preferred high T_(g) polymeric additives arebiodegradable and derived from renewable resources. Exemplary preferredhigh T_(g) polymeric additives include cellulose acetate, cellulosepropionate, cellulose butyrate, cellulose acetate propionate, celluloseacetate butyrate, cellulose propionate butyrate, terpene resins androsin and rosin esters derived from tree sap.

A preferred fourth composition which can be used for preparing theblocking reducing layer includes a lactic acid residue containingpolymer and a semicrystalline polymeric additive. Preferredsemicrystalline polymeric additives will have a melting point above 90°C. and more preferably above 120° C. The most preferred semicrystallinepolymeric additives are biodegradable and derived from renewableresources. Preferred semicrystalline polymeric additives includealiphatic polyester with melting points above 90° C. Exemplary preferredsemicrystalline polymeric additives include poly(hydroxy butyrate),poly(hydroxy butyrate-co-hydroxy valerate), polybutylene(succinate),polybutylene(succinate-adipate copolymer), polyethylene(succinate), andpolyethylene(succinate-adipate copolymer). It is believed thatpoly(glycolide), poly(lactide), or the stereocomplex of poly(L-lactide)and poly(D-lactide) might also be suitable for use as antiblockingagents.

It is understood that the semicrystalline polymeric additives should bepresent in an amount of between about 5-70% by weight of blockingreducing layer, more preferably between about 10 and 50% by weight. Inthe case of additive such as polyhydroxybutyrate (PHB) polymers andpolyhydroxy butyrate/valerate copolymers (PHBV), it is preferred thatthey be present in an amount of about 100 by weight of blocking reducinglayer.

Examples of polyhydroxybutyrate (PHB) polymers and polyhydroxybutyrate/valerate copolymers (PHBV) are described in U.S. Pat. No.4,393,167, Holmes et al., issued Jul. 12, 1983, and U.S. Pat. No.4,880,592, Martini et al., issued Nov. 14, 1989, and U.S. Pat. No.5,391,423, Wnuk et al., issued Feb. 21, 1995, these patents beingincorporated herein by reference. Polyhydroxy butyrate/valeratecopolymers are commercial available from Ceneca Corp. under thetradename Biopol™. The Biopol polymers are produced from thefermentation of sugar by the bacterium Alcilagenes eutrophus. PHBVpolymers are currently produced with valerate contents ranging fromabout 5 to about 25 mole percent. Increasing valerate content decreasesthe melting point, crystallinity, and stiffness of the polymer. Anoverview of Biopol technology is provided in Business 2000+, (Winter1990), incorporated herein by reference.

Examples of polymers of polybutylene(succinate),polybutylene(succinate-adipate copolymer), polyethylene(succinate), andpolyethylene(succinate-adipate copolymer) are available under the nameBionolle® from Showa Highpolymer Co., Ltd.

Without being bound by theory, it is believed that the limitedcompatibility of the anti-blocking agent in the blocking reducing layermay be partly responsible for enhancing the anti-blockingcharacteristics thereof.

A preferred fifth composition which can be used for preparing theblocking reducing layer includes a rapidly crystallizable polymer havinga high melting temperature (T_(m)). Preferably, it also exhibits a lowglass transition temperature (T_(g)). It is believed that the rapidcrystallization will facilitate processing by reducing or preventingsticking and blocking during film handling.

Polymer compositions having a low T_(g) and high T_(m) are desirablebecause they can provide rapid crystallization after processing.Typically, in order for a polymer composition to exhibit rapidcrystallization, it needs to be well below its T_(m). Under normalprocessing condition, the polymer composition should therefore have aT_(m) above about 80° C. and below about 200° C., and preferably belowabout 170° C. A T_(m) of 80° C. is believed to be high enough so that apolymer can crystallize during typical blown film production. A T_(m) of80° C. or higher will also provide excellent blocking performance undertypical use and storage conditions. The upper limit on the T_(m) isdetermined by providing a composition which can be readily processablein line with a biodegradable polymer such as a lactic acid basedpolymer. Polymer compositions having a T_(m) above 200° C. generallyrequire processing conditions which make it difficult to provide on thesame line as, for example, a plasticized polylactide polymer, even withthe use of a multilayer die with distinct heating sections.

The glass transition temperature of the polymer composition should berelatively low in order to provide desired performance under certainapplications. For example, a low glass transition temperature isparticularly important for use in outdoor applications, such as lawn andleaf disposal bags. It has been observed that a low T_(g) in the outerlayer can help to strengthen the bag properties which otherwise are bornentirely by the core layer. In most applications, the T_(g) should bebelow about 10° C., and preferably below 0° C., and more preferablybelow −10° C.

In the present invention, a polymer composition layer is considered tobe semicrystalline it if exhibits a net melting endotherm of greaterthan 10 J/g of polymer when analyzed by a different scanning calorimeter(DSC). To determine whether a polymer composition layer issemi-crystalline, it can be tested in a differential scanningcalorimeter, such as by Mettler. The details of performing a test ofcrystallinity are known to those skilled in the art and are identifiedin U.S. patent application Ser. No. 08/110,394, filed on Aug. 23, 1993,the complete disclosure being incorporated herein by reference.

The extent of crystallinity should be sufficient to provide an outerlayer having a crystallinity of at least about 10 J/g based on theweight of the outer layer only, or roughly 3 J/g based on the weight ofthe film having a layered cross-section of 15/70/15 by weight of eachlayer. This is believed to be sufficient to give excellent blockingresistance. Preferably, the crystallinity of the outer layer can begreater than 30 J/g. In most applications, the crystallinity of theouter layer will be less than 100 J/g.

While not being bound by theory, it is believed that the most preferredfifth composition will exhibit physical compatibility with the corelayer but will not exhibit thermodynamic miscibility. Thus, a blend ofcore layer and outer layer may exhibit two glass transitiontemperatures, each being approximately equal to the glass transitiontemperature of each of the individual compositions. The low T_(g) of thepreferred outer layer compositions would be an indicator that thestructure is significantly different than poly(lactide), makingthermodynamic miscibility less likely. Typically, the low T_(g)materials will have a higher proportion of —CH2— units in the backbonethan will poly(lactide). The difference in structure is used toadvantage by selecting a plasticizer for the core layer which is morecompatible with poly(lactide) than with the outer layer polymer. Thisacts to keep the plasticizer in the polymer core layer, preventingmigration of plasticizer. Migration of the plasticizer into the outerlayer is undesirable because it raises the T_(g) of the core layer,reducing low temperature performance, and because it can lead to “oily”surfaces which are a detraction to product acceptance. It is believedthat crystallization of the outer layer is also beneficial for reducingthe migration of plasticizer.

The physical compatibility is preferred so that the layers will form areasonable bond at the interface, without having to result to costly“tie” layers with their added complexity. Physical compatibility is amore qualitative judgment, but typically physical compatibility isobserved by lack of macroscopic phase separation, lack of a “cheesy”texture, and by tensile properties (such as tensile stress at break,tensile stress at yield, and elongation at yield) which are at least asgood for the blend as they are for the minimum of each of the two purecomponents. A blend, such as 50/50 by weight, will exhibit tensileproperties which are higher than the lower of the two pure components.Physical compatibility is a complex function of molecular weight andchemical structure, but will occur if two polymers are not toodissimilar. Aliphatic polyesters, and copolymers of aliphatic polyesterswith other components, are expected to exhibit a certain degree ofphysical compatibility. An estimate of the minimum ester content toachieve physical compatibility is to have at least one ester group per200 AMU of polymer unit.

The preferred polymers to meet these criteria are generally based onaliphatic polyesters, produced either from ring opening reactions orfrom the condensation of acids and alcohols. Typically, diols anddiacids are reacted to form an aliphatic polyester by condensationpolymerization. Often this limits the potential molecular weight to anumber average molecular weight of less than 30,000, although, in somecase, it may be as high as 50,000. To achieve higher molecular weightsis generally very difficult. This molecular weight limit tends to resultin polymers with poor tear strength, which is a critical property forfilm bag applications. Thus, these polymers, on their own, may haveinsufficient tear strength for a commercially acceptable film bag.

Aliphatic polyesters based on diacids and diols are availablecommercially and are generally preferred. The aliphatic polyesters withan even number of carbons in the diacid generally have a morecrystalline nature than those with an odd number of carbons. Thepreferred aliphatic polyesters comprise the reaction products of aC₂-C₁₀ diol with oxalic acid, succinic acid, adipic acid, suberic acid,sebacic acid, or mixtures and copolymers thereof. More preferredpolyesters include polyethylene(oxalate), polyethylene(succinate),polybutylene(oxalate), polybutylene(succinate),polypentamethyl(succinate), polyhexamethyl(succinate),polyheptamethyl(succinate), or polyoctamethyl(succinate), mixtures orcopolymers thereof, or copolymers of these with adipic acid. Especiallypreferred are polyethylene(succinate),polyethylene(succinate-co-adipate), polytubylene(oxylate),polybutylene(succinate), polybutylene(succinate-co-adipate),polybutylene(oxylate-co-succinate and/or adipate), and mixtures thereof.The polybutylene terminology in this case refers to the condensationproduct of 1,4 butane diol and polyethylene terminology refers to thecondensation product of 1,2 ethan diol, also know as ethylene glycol. Toensure reasonable rates of crystallization and sufficiently high Tm, itis anticipated that any copolymers will contain at least 70 mole % ofthe primary diacid (on a diacid basis). The aliphatic polyesters mayalso contain units derived from non-aliphatic diacids, or esters, suchas terephthalic acid or methyl terephthalate. The condensation productsof diacids with polyether diols may also be useful as outer layers inthe multilayer film application.

An exemplary preferred polymer is a polybutylene succinate homopolymersold under tradename Bionelle 1000™ and is available from ShowaHighpolymer Co., Ltd. It is believed thatpolybutylene(succinate-terephthalate copolymer) andpolybutylene(adipate-terephthalate copolymer) will be useful in formingthe blocking reducing layer.

The Core Layer

In a preferred composition, the core layer will have a T_(g) below 20°C. and more preferably below 10° C. IN the case of a core layercontaining a polymer composition including a lactic acid residuecontaining polymer, reduced T_(g) can be provided by a plasticizer levelof about of 20 wt-% or more. Furthermore, when the polymer compositionincludes a lactic acid residue containing polymer, it is preferred thatthe lactic acid residue containing polymer be non-crystallizablecompositions such as those described in U.S. patent application Ser.Nos. 08/110,424 and 08/110,394, filed on Aug. 23, 1993, U.S. Pat. Nos.5,84,881 and 5,536,807 respectively, the entire disclosures beingincorporated herein by reference. Generally, this means at least 15% ofthe minor otpical isomer of lactic acid residue is present.Alternatively, this means that the lactic acid residues are present inan optical purity of no more than 85%.

Applicants believe that it may be possible to modify the core layer witha blocking reducing modifier to reduce migration of plasticizer andthereby reduce blocking. An exemplary blocking reducing modifier isperoxide. Example 5 demonstrates the use of a peroxide blocking reducingmodifier in a PLA and plasticizer composition. Without being bound bytheory, it is believed that the blocking reducing modifier functions bymodifying the core layer, possibly by reaction, so as to discourageplasticizer migration out of the core layer. It is believed that theblocking reducing modifier may bond the plasticizer to the core layer.In the case where peroxide is used as the blocking reducing modifier, itis preferably provided at a concentration of about 0.05 to about 0.5percent by weight.

The core layer of the multilayer structure should be sufficientlyflexible to be rolled or folded for packaging, to be useful for purposeintended. Preferably, the first layer should have sufficient flexibilityto allow it to be folded over onto itself without cracking at thecrease. It is preferred that a multilayer film according to the presentinvention would have a tensile modulus of less than 75,000 psi at 23° C.when tested according to ASTM D-882 method A-3.

Pigments or color agents may also be added as necessary. Examplesinclude titanium dioxide, clays, calcium carbonate, talc, mica, silica,silicates, iron oxides and hydroxides, carbon black and magnesium oxide.

Applicants have found that the presence of residual catalysts in thelactic acid residue containing polymer structure significantly affectsthe stability thereof during processing. Accordingly, the catalyst levelcan be controlled as described in U.S. Pat. No. 5,338,822, which isincorporated herein by reference.

Forming the Multilayer Structure

The multilayer structure of the present invention can be manufactured bya variety of techniques, including coextruding blown or cast film. Thepreferred technique is coextrusion of blown film where all layers areextruded concurrently and are applied to each other either soon beforeor soon after they leave the co-extrusion die. A second preferredtechnique is coextrusion blow molding, for the production of articles.These techniques allow all the layers of the multilayer structure to besimultaneously extruded, stretched and combined. Other processingtechniques used to prepare films, sheets, laminates and the like can beused in the present invention. Many techniques are described, forexample, in Encyclopedia Of Polymer Science And Engineering, 2ndEdition, John Wiley and Sons, 1986, which is incorporated herein byreference.

As demonstrated by Example 4, it is important to match the viscosity ofeach layer in order to provide a structure wherein having a maximumthickness of the flexible core layer relative to the blocking reducinglayers. Maximizing the core layer is believed to provide a maximum tearstrength. In addition, it is believed that viscosity matching increasesthe compatibility between the layers. Viscosity matching can beaccomplished by a variety of techniques including modifying thetemperature of the die or individual polymer melt streams and/oraltering the composition of the extrusion material.

When preparing the multilayer structure by blown film coextrusion orcoextrusion blow molding, it may be desirable to inject the plasticizerinto the polymer composition which forms the core layer while it is inthe extruder. This avoid having to handle pellets having a low T_(g)which would tend to stick together.

It should be appreciated how the orientation of the layers can bemodified to provide desired physical properties. For example, thephysical properties of a multilayer film prepared by blown filmcoextrusion can be altered by adjusting the blow-up-ratio (BUR) which isdefined as the ratio of the diameter of the film bubble to the diediameter. For purposes of providing a bag or wrapper, it is typicallydesirable to provide a BUR of between about 1 and 5, and should bedetermined based upon the desired balance of properties in the machinedirection and the tensile direction. For a balanced multilayer film, apreferred BUR of about 3:1 is appropriate. It is recognized that it maybe desirable to have a “splitty” film which easily tears in onedirection such as the machine direction. It is expected that a BUR ofabout 1:1 to 1.5:1 should provide such a multilayer structure.

As demonstrated by the data in Example 8, it is understood thatmigration of plasticizer from the core layer into the blocking reducinglayers can be minimal over time. Without being bound by theory,Applicants hypothesize that plasticizer begins migrating into theouter/non-plasticizer containing layers, and that the presence ofplasticizer allows the interfaces between the core layer and thenon-plasticizer containing layers to crystallize which discourages orretards further migration of plasticizer. It is believed that byinducing crystallization at the interface, or in the boundary area inthe non-plasticizer containing layers, properties including vapor andmoisture diffusion can be altered.

Barrier properties, such as oxygen and water permeability, can also beaffected by crystallizing and/or orienting the film. If the desiredpermeability cannot be achieved through processing, then a layer ofadditional polymer may be introduced to provide the desired barrierproperties. For example, a thin layer of polyethylene, polypropylene,polyethylene terephthalate (PET), or other polymer with a waterpermeability of less than about 5 g-mil/100 in²-day, could be added toreduce the water permeability of a lactic acid residue containingpolymer based film. It has been found that the moisture vaportransmission rate for a multilayer film, such as in Example 2, is about25 g-mil/100 in²-day.

The oxygen permeability of lactic acid residue containing polymer basedfilms has not been reported, but it is estimated to be about 2×10⁻¹¹cc-cm/(cm²-sec-cm Hg). A thin layer of PVOH, EVOH, PAN, or other polymerwith oxygen permeability of less than about 2×10⁻¹²cc-cm//(cm²-sec-cmHg) could be added to reduce the oxygen permeabilityof a lactic acid residue containing polymer based film.

EXAMPLES Example 1 Example Showing Inverse Relationship Between TearResistance and Blocking Resistance and Direct Relationship BetweenPlasticizer Level and T_(g)

A Leistritz 34 mm twin screw extruder was used to compound a mixture ofcomponents described below. The extrudate was cooled in a water bath andchopped into pellets. The pellets were then coated with 0.1% Ultra-Talc609 to prevent agglomeration, dried at 30° C., and extruded through aflat die to form a structure for property testing.

The twin screw extruder was operated with zone 1 (pellet feed zone) at150° C., zone 2 at 160° C., zones 3-6 at 170° C., zones 7-8 at 165° C.,and zones 9-11 at 160° C. The screw speed was set at 200 rpm. Pellets ofpolylactide (PLA) polymer, which is a copolymer of lactide with 0.35 wt.percent of epoxidized soybean oil and having a number average molecularweight of 104,000 and a D-level of 11%, available from Cargill, were fedinto zone 1 at a rate of 123 g/min using an AccuRate feeder. Aplasticizer, acetyl tri-n-butyl citrate from Morflex, Inc. was injectedinto zone 3 of the extruder at ambient temperatures using a liquidinjection system. The plasticizer was fed in at a rate of 31.5 g/minproviding a composition of 20.4% plasticizer.

The compounded mixture containing 20% plasticizer was then dry blendedwith sufficient amounts of the PLA used in the initial compounding(Mn-104,000, D-level of 11%) to obtain mixtures of 0, 5, 10, 15, and 20%plasticizer. These mixtures were then extruded on a ¾″ Killion extruder,through a six inch flat die, into film having a 3.25 mil thickness(0.00325″). The Killion extruder operated with zone 1 at 280° F., zone 2at 290° F., zone 3 at 300° F., and zone 4, the adapter, and the die allat 315° F.

The glass transition temperature (T_(g)) for each film was determinedusing Differential Scanning Calorimetry (DSC) according to procedureknown in the art. A typical procedure includes taking a small sample ofthe film (5-20 mg) and placing it in a sealed capsule. The capsule isloaded in to the DSC and cooled to a temperature well below the expectedT_(g), e.g., −100° C. The sample is then heated at a rate between 5°C./min and 20° C./min and the heat input relative to a blank referencecell is recorded. The glass transition temperatures are evaluated, andrecorded as the midpoint of the typical sigmoidal curve. The sample isevaluated on the first upheat of the DSC, to avoid any mixing of thesample phases.

The films were then aged and tested for tear propagation resistance andfor blocking resistance. The tear propagation resistance test wasconducted according to ASTM Method D 1922-89. The blocking resistancetest involved placing two films on top of each other and placing thereona 400 gm weight with a 2.2 in² contact area. This was left in atemperature controlled environment for 2.0 hours at 50° C. and checkedfor blocking. The blocking scale for this test ranges from 0 for noblocking to 5 for complete fusion of the two layers. The results of thetear propagation resistance and blocking resistance tests are providedin Table 1.

TABLE 1 Normalized Elmendorf Percent Elmendorf Tear (gm_(f)) Blocking(gm_(f)/mil) Plasticizer MD (avg) TD (avg) Level Tg (° C.) MD TD  0 1814 1 60 5.5 4.3  5 19.7 21.8 2 52 6.1 6.7 10 31.2 26.33 4 41 9.6 8.1 15704 816 5 30 220 250 20 1,510 1600+ 5 19 460 490

The results in Table 1 indicate that blocking resistance is inverselyrelated to tear propagation resistance for single layer, plasticizedfilms of polylactide. The results further indicate that the glasstransition temperature (T_(g)) is directly related to the amount ofplasticizer therein. It is a discovery of the present invention thathighly plasticized films of polylactide, while providing the desiredproperties of low T_(g) and high tear strength, develop severe blockingproblems. It should be appreciated that a blocking level of 1 indicatesthat there was substantially no blocking which indicates that there wasat most minor adhesion but that the films could be pulled apart withoutsignificant deformation.

The normalized Elmendorf tear values are used to get approximate tearvalues of a 1 mil film.

Example 2 Example Showing Multilayer Film with Good Blocking Resistanceand Good Tear Propagation Resistance

A multilayer film was produced on a 10″, four layer, StreamlinedCoextrusion Die (SCD) blown film die manufactured by BramptonEngineering. Layer configuration of the die is as follows from outsideto inside layers of the die, A/B/D/C. Three 3½″ David Standard extrudersfed the A, D, and C layers while a 2½″ David Standard extruder fed Blayer. The process line also utilized a Brampton Engineering rotatingair ring for polymer cooling. Layers B and D contained PLA (Mn=103,000,D-level of 11%) plasticized with 20% Citroflex which was compounded asdescribed in Example 1. Layers A and C contained PLA (Mn=66,000, D-levelof 3%) dry blended with 10% Biopol D300G, supplied by ZenecaCorporation. Layer ratios for the film were A-19%, C=21%, combination ofB and D=60% of the total film structure. The thickness of the filmproduced was 2.25 mil (0.00225″). The processing conditions for the filmare provided in Table 2.

TABLE 2 Extruder A Extruder B Extruder C Extruder D Zone 1   300   300  300   300 Zone 2   310   310   310   310 Zone 3   320   320   320  320 Zone 4   340   330   330   340 Zone 5   340   340 Scn Chngr   330  330   330   330 Adapter 1   330   330   330   330 Adapter 2   330  330   330   330 Adapter 4   330   330   330   330 Die 1   330   330  330   330 Die 2   330   330   330   330 Die 3   330   330   330   330Pressure 1,280 1,670 1,640 1,310 Melt Temp   336   338   338   339 ScrewSpd   14   50   48   12 Amps   50   40   45   120 Line Spd 122 fpm NotesPLA/Biopol Plasticized Plasticized PLA/Biopol blend PLA PLA blend Note:Temperatures in table 2 are given in ° F.

Tear propagation resistance and blocking resistance testing wasconducted on the multilayer film according to the procedure described inExample 1. The test results are provided in Table 3. Additionally, themultilayer film exhibited no sign of blocking when tested at 70° C. for24 hours.

TABLE 3 Normalized Elmendorf Tear Elmendorf Tear (gm_(f)) (gm_(f)/mil)MD (avg) TD (avg) Blocking Level MD TD 112 242 0 @ 70° C. 50 107

The results in Table 3 indicate that the non-plasticized outer layersprevent blocking while the plasticized inner layers provide the tearresistance of the film.

The normalized Elmendorf tear, although not recommended by ASTM, can beused to provide an estimate of the tear strength of a 1 mil film.

Example 3 Example Showing Bag Making Ability on a Commercial Line

The multilayer film prepared in Example 2 was converted into bags usingan in-line bag machine manufactured by Battenfeld Gloucester EngineeringCo., Inc. downstream of the extrusion line nips. This exampledemonstrates the feasibility of making bags on a commercial scale line,and at commercial speeds, from multilayer films according to the presentinvention.

Example 4 Example Showing Importance of Viscosity Matching

Film samples were run on a 6″ 7-layer SCD blown film die manufactured byBrampton Engineering, Inc. with a die gap of 0.060″ and a Uni-Flo airring for film cooling. Labeling of the die layers are from outside toinside A, B, C, D, E, F, G. Layers B, C, D, E, and F were fed by five 30mm Brampton extruders. Layers A and G were fed by two 45 mm Bramptonextruders. In making the film samples layers A and G were filled with0.85MI (melt index) polyethylene and cooled down to 100° F. to “freeze”the layers and effectively make the die a five-layer system.

Layers C, D, and E contained PLA (Mn=102,000) plasticized with 20%Citroflex which was compounded as described in Example 1. Layers B and Fcontained PLA dry blended with 10% Biopol D300G, supplied by ZenecaCorporation. Two different molecular weight PLA were used in the blendsfor the B and F layers. This allowed us to determine the effects ofviscosity on the ability to minimize the film's outer layer percentage.Mn's of 66,000 and 102,000 were utilized. Rabinowitsch correctedviscosities at 177° C. and 200 s⁻¹ for the core layer material wasapproximately 300 Pa·s, and for the 66,000 Mn material it was 500 Pa·s,and 700 Pa·s for the 102,000 Mn material.

In running the films the outer film layers were run to the minimumthickness percentage as possible while maintaining consistent flow andgood film appearance. Additionally it was found that when the outerlayers were run to their minimum the bubble/tube stability would bejeopardized and the bubble would collapse, resulting in a less of filmproduction. With the Mn of the PLA in the outer layers at 66,000, layerratios of both B and F lines were obtainable at 5-10% of the total filmstructure each. This led to the plasticized layers constituting 80-90%of the total film structure. When the Mn of the PLA in the outer layerswas at the 102,000 level the lowest ratio for B and F layers obtainablewas 20% each. This resulted in the plasticized layers constituting 60%of the total film structure. As the outer layer percentages were reducedduring this trial, there were initially flow instability lines apparenton the film at ratios of less than 20% for B and F layers. As thepercentages approached 18% the bubble collapsed. Applicants believe thiswas due to holes generated in the film from the flow instability at thelayer interface.

Viscosity matching allows production of films with maximum thickness ofthe central flexible layer, which is believed to give a maximum tearstrength. Viscosity matching can be either through compositionalcontrol, as in the present example or through use of a multi-temperaturedie.

Processing conditions for the films are provided in Table 4.

TABLE 4 A line B line C line D line E line F line G line Zone 1 100 290290 290 290 290 100 Zone 2 100 340 320 320 320 340 100 Zone 3 100 350330 330 330 350 100 Adapter 100 350 340 340 340 360 100 Die 1 350 Die 2350 Die 3 350 Die 4 350 Die 5 350 Die 6 350 Material 0.85 MI PLA/Plasticized Plasticized Plasticized PLA/ 0.85 MI PE Biopol PLA PLA PLABiopol PE Blend Blend

Example 5 Example Showing Disadvantage of Having no Anti-Blocking Agentin the Outer Layers

Films were prepared on the equipment described in Example 4 under thesame processing conditions. Plasticized PLA was compounded as in Example1 with the exception that 0.1 wt. % Luperco 233-XL supplied by ElfAutochem was added to compound. This was then fed into C, D, and Elayers. PLA and PLA blended with Biopol D300G were utilized in the B andF layers to determine the effects of adding anti-blocking agents to theouter layers of the films. The PLA used had an Mn of 66,000. Thecomposition containing the Biopol consisted of 95% PLA and 5% BiopolD300G. The Biopol D30OG was utilized as an anti-blocking agent in thisinstance.

During extrusion of the films, the sample containing only PLA in layersB and F showed blocking at a level 3, as described in Example 1, uponcollapsing the bubble at the take-off nip. The film containing the blendof PLA and Biopol in the B and F layers showed blocking at a level 0upon collapsing the bubble. Additional testing of the film for blockingresistance was completed in accordance with the test described inExample 1 except that the films were tested for 24 hours.

TABLE 5 Nip Outer Layer Blocking Blocking Level Composition Level inTest PLA 3 5 PLA/Biopol Blend 0 1

Careful selection of the outer-layer material is important for optimalblocking resistance.

Example 6 Example Showing Use of Non-PLA Materials in Multi-Layer FilmStructures

Films were produced using the equipment and processing conditionsdescribed in Example 4 to produce structures containing materials otherthan PLA as the base material for one or more of the film layers. Whenplasticized PLA was utilized it was prepared in accordance to the methoddescribed in Example 1. Again only the B, C, D, E, and F layers of thedie were utilized to produce the films. In all of these films there wasan attempt to make a flexible core layer surrounded by rigid,non-blocking outer layers.

TABLE 6 Material Sample Layer B Layer C Layer D Layer E Layer F 1 PLAPVOH PVOH PVOH PLA 2 PLA Polyethylene Polyethylene Polyethylene PLA 3Biopol Plasticized Plasticized Plasticized Biopol PLA PLA PLA 4 EVOHPlasticized Plasticized Plasticized EVOH PLA PLA PLA

In all of the above cases except for sample #3 there was poor adhesionbetween the PLA layers and the “other” material layers. In the case ofsample #3 a film with 5% layer ratios for B and F layers wasaccomplished. This film showed no blocking at the haul off nip and alsodemonstrated a blocking level of 0 when tested in accordance to the testdescribed in Example 1. The PVOH was supplied as VINEX 2144 by AirProducts And Chemicals, Inc., the PE was a LLDPE (grade 2045) suppliedby Dow, and the EVOH was supplied by Eval Corporation.

Example 7 Example Showing Effects of Layer Ratios on Physical Properties

Films were processed on the equipment described in Example 4 to producefilms with varying layer ratios. The outer layers, layers B and F,utilized PLA dry blended with 10% Biopol D300G. The inner layers of thefilm, layers C, D, and E, utilized PLA compounded with 20% Citroflexplasticizer as described in Example 1. The following films were producedwith thickness ranging from 1.5-1.75 mil (0.0015″-0.00175″).

TABLE 7 B Layer C, D, E, Combined F Layer Sample # Ratio (%) Layer Ratio(%) Ratio (%) 1 25 50 25 2 20 60 20 3 15 70 15 4 10 80 10 5  5 90  5

The films were conditioned in a 50% relative humidity chamber at 20-25°C. and tested for tear propagation resistance according to ASTM D-1922,tensile properties according to ASTM D-882 method A3, and impactresistance according to ASTM D-3420 with the results provided in Table8.

TABLE 8 Tear Ultimate Yield Resistance Normal- Tensile Ultimate TensileYield Elong. Outer (g_(f)) ized (psi) Elong. (%) (psi) (%) Tensile Layer(Avg) (Avg) Tear (Avg) (Avg) (Avg) (Avg) (Avg) (Avg) (Avg) (Avg)Strength? (%) MD TD (gm_(f)/mil) MD TD MD TD MD TD MD TD MD TD 25 36.547.5 23 29 5,474 4,878 296 342 4,536 3,738 5.4 4.6 84 81 231 20 40.546.5 25 28 5,670 4,813 329 317 3,838 3,418 5.2 6.2 74 55 643 15 49 64.531 40 5,189 4,476 351 313 3,544 2,673 5.6 6.1 63 44 660 10 56.5 89.S 3556 S,053 4,299 340 343 2,470 2,087 5.4 4.5 46 46 812  5 94.3 128 58 804,921 4,845 367 422 1,916 1,608 6.2 9.8 31 16 1,236

The data in Table 8 demonstrates the effect the layer ratios have on thetear propagation resistance, the yield strength, and the impactstrength. Increasing the thickness of the inner, flexible layer, anddecreasing the thickness of the outer layers, provides high tearresistance and impact strength, although yield is reduced. The data alsodemonstrates that the layer ratios do not have much of an effect on theultimate tensile strength, the ultimate elongation, or the yieldelongation.

The normalized tear force is provided as an estimate for a 1 mil film,although it is preferred to test a 1 mil film directly.

Example 8 Example Showing Non-Migratory Plasticizer in Coextruded Film

The film produced in Example 2 was run on a DSC to determine the T_(g)of the two distinct types of PLA present in the structure. Evaluatingthe shift in T_(g) over time provides an indication of plasticizermigration or lack of migration. An upward shift in the core(plasticized) layer's T_(g) would indicate migration of plasticizer outof the core layer. This would correspond to a downward shift of theouter layer T_(g) as the plasticizer migrated into these layers. The DSCtest results are provided in Table 9.

TABLE 9 T_(g1) (° C.) T_(g2) (° C.) DSC Test Date for core layer forouter layer 12/27/94 18.3  53.45 01/04/95 18.98 54.07 01/20/95 17.4953.05 02/03/95 15.24 56.30 08/17/95 16.76 53.77

The data in Table 9 demonstrates that there is no appreciable change inthe T_(g) of the layers of the film structure over an 8 month period.This indicates a lack of migration of the plasticizer over time for thetested multilayer film structure.

Example 9 Example Showing Compostable Properties of Coextruded Film

The multilayer film of Example 2 was tested to determine the rate andextent of degradation in a compost environment. For comparativepurposes, sample of kraft paper and cellulose were similarly tested toevaluate the rate and extent of degradation in a compost environment.The kraft paper was from a typical grocery bag and the cellulose wasmicrocrystalline cellulose from Avicel.

For all three samples, a modified version of ASTM D5338-92 test wasperformed according to ASTM D5338-92 except that a constant temperatureof 58° C. was provided in order to more conveniently approximate naturalcomposting conditions. The amount of material biodegraded was calculatedbased upon measuring the amount of carbon dioxide evolved therefrom.

Test results were plotted in the graph of FIG. 3 as “Cummulative %Biodegradation (CO2-C)” as a function of time. The graph demonstratesthat the multilayer film of Example 2 degrades at a rate and to anextent fairly close to cellulose and kraft paper.

Comparative Example 1

Monolayer films having a thickness of 2 mil were prepared frompolybutylene(succinate) and from polybutylene(succinate-adipatecopolymer). The polymer samples are available as Bionolle® 1001 and 3001from Showa Highpolymer Co., Ltd. The films were blown using a 1″ Killiondie and 1″ Killion single screw general purpose extruder with 3:1compression ratio and 24:1 L:D. A single tip air ring was used toprovide bubble inflation. Throughput was about 8 lb/hr.

The films were tested for Elmendorf tear following ASTM D1922-89, withthe following results, all in grams-force (gm_(f)).

polybutylene (succinate- polybutylene (succinate) adipate copolymer)Temperature MD TD MD TD 23° C. 31 49 34 54 10° C. 26 35 29 43  0° C. 3134 30 44

The polybutylene(succinate) film exhibited a T_(g) of −37° C. and 47.4J/g crystallinity, with a peak melting point of 113° C. Thepolybutylene(succinate-adipate copolymer) film had a Tg of −45° C. and34 J/g crystallinity, with a melting temperature of 94° C. The strength,however, is insufficient for commercial use as a lawn refuse bag.

Example 10

Two films were prepared on a multilayer blown film line to prepare aA-B-A composition. The “A” material was fed using a ¾″ Brabender generalpurpose extruder, the “B” material was fed using a 1″ Killion generalpurpose extruder, and the die was a 1″ Killion 3-layer die side fed,with a 0.030″ die gap. The “A” material was fed at a rate of 3.6 lb/hrand had a melt temperature of 365° F. The “B” material was fed at 8.4lb/hr and had a melt temperature of 325° F. A single lip air ring wasused for inflation and the film take-off speed was about 10-30 ft/min.

The core layer in each case consisted of poly(lactide) with a numberaverage molecular weight of about 100,000 and which included 0.35 wt %of epoxidized soybean oil in the polymerization. The overall opticalcomposition was 85% S-lactic acid residuals and 15% R-lactic acidresiduals, from lactide. The core layer additionally contained 20 wt %of the plasticizer acetyl ti-n-butyl citrate, available as Citroflex™A-4 from Morflex, Inc. The outer layer for film one was a blend of 90 wt% poly(lactide) and 10 wt % poly(hydroxy butyrate-co-hydroxy valerate),called PHBV for short, available as Biopol™ D300G from ICI.

For the first multilayer film, the outer layer included a polylactidepolymer containing 0.35 wt % epoxidized soybean oil added prior topolymerization in a batch reactor and having a number average molecularweight of about 90,000 and an overall optical composition including 95%S-lactic acid residuals and 5%. R-lactic acid residuals.

For the second multilayer film, the outer layer included apolybutylene(succinate) polymer, available as Bionolle 1001 from ShowaHighpolymer Co., Ltd.

Both multilayer films were 2 mil thick and had a layer ratio of 15/70/15percent by weight. Table 10 shows the Elmendorf tear results in gm_(f),according to ASTM D1922-89, for each multilayer film wherein the filmsare identified by their outer layer.

TABLE 10 poly(lactide)/PHBV polybutylene (succinate) blend outer layerouter layer [3054-8-2] Temperature MD TD MD TD 23° C. 93 150 172 176 10°C.  80  79  0° C. 12  17  47  54

The tests show that each film exhibits good tear strength at 23° C. Thelow temperature tear strength of the film with an outer layer ofpolybutylene(succinate) was superior to the film with an outer layer ofpoly(lactide)/PHBV. It is believed that the low T_(g) (estimated to beless than −30° C.) of the outer layer for film two assists in givinggood properties at low temperature.

Example 11

Two films, each 2 mil thick, were prepared on a blown film lineaccording to the procedure described in Example 10. Each film included acore layer of polylactide with 85-88% S-lactic acid residuals and 12-15%R-lactic acid residuals (from lactide) to form an amorphous film withnumber average molecular weight of 85,500 for film 1 and 106,000 forfilm 2. The polylactide included 0.35 wt % of epoxidized soybean oilincluded during the polymerization, carried out in a batch reactor. Thepolymer was blended with 25 wt % of a plasticizer which was tri-n-butylcitrate, available as Citroflex™ C-4 from Morflex, Inc. The Elmendorftear properties are shown in the table 11.

TABLE 11 polybutylene (succinate) polybutylene (succinate) outer layer(film 1) outer layer (film 2) Temperature MD TD MD TD 23° C. 186 304 139147 10° C.  72 110  0° C.  47  66  76  75

Film 1 exhibited two Tg's, one at about −35° C. corresponding to theouter layer and one at 8.6° C. for the plasticized poly(lactide) corelayer. The outer layer exhibited a Tm of 109° C. with 16.7 J/g on awhole film basis, corresponding to 56 J/g on an outer layer basis. DSCresults are not available on film 2.

Each of these two samples shows good tear strength and had good blockingresistance to at least 60° C.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that different alternatives,modifications, variations, and uses will be apparent to those skilled inthe art in view of the foregoing description. Accordingly, the inventionis not limited to the embodiments presented herein.

What is claimed:
 1. A compostable multilayer film comprising: (a) corelayer comprising a polylactide polymer, the core layer having a T_(g)below about 20° C., and having first and second opposed surfaces; (b)first blocking reducing layer covering the first surface of said corelayer; and (c) second blocking reducing layer covering the secondsurface of said core layer; wherein at least one of said blockingreducing layers comprises a semicrystalline polymer compositioncomprising an aliphatic polyester polymer.
 2. The compostable multilayerfilm according to claim 1, wherein the blocking reducing layercomprising a semicrystalline polymer composition has a crystallinity ofgreater than 10 J/g as determined by a differential scanningcalorimeter.
 3. The compostable multilayer film according to claim 1,wherein the blocking reducing layer comprising a semicrystalline polymercomposition has a crystallinity of greater than 30 J/g as determined bya differential scanning calorimeter.
 4. The compostable multilayer filmaccording to claim 1, wherein the aliphatic polyester polymer comprisesa polymer selected from the group consisting of polyethylene(oxalate),polyethylene(succinate), polybutylene(oxalate), polybutylene(succinate),polypentamethyl(succinate), polyhexamethyl(succinate),polyheptamethyl(succinate), polyoctamethyl(succinate),polyethylene(succinate-co-adipate), polybutylene(succinate-co-adipate),polybutylene(oxylate-co-succinate), polybutylene(oxylate-co-adipate),and mixtures thereof.
 5. The compostable multilayer film according toclaim 1, wherein the semicrystalline polymer comprises polybutylenesuccinate homopolymer.
 6. The compostable multilayer structure accordingto claim 1, wherein said compostable multilayer film has abiodegradability value of about 20 percent or higher after 40 daysaccording to ASTM D5338-92, modified to test at about 58° C.
 7. Thecompostable multilayer structure according to claim 1, wherein saidcompostable multilayer structure has a biodegradability value of about50 percent or higher after 40 days according to ASTM D5338-92, modifiedto test at about 58° C.
 8. The compostable multilayer film according toclaim 1, wherein the polylactide polymer is a copolymer prepared byreacting lactide monomer with non-lactide or non-lactic acid monomer. 9.The compostable multilayer structure according to claim 8, wherein thenon-lactide monomer is epoxidized multifunctional oil.
 10. Thecompostable multilayer film according to claim 1, wherein at least oneof the blocking reducing layers includes an antiblocking polymeradditive selected from the group consisting of poly(hydroxybutyrate) andpoly(hydroxybutyrate-co-hydroxyvalerate).
 11. The compostable multilayerfilm according to claim 1, wherein the core layer is peroxide modifiedto reduce plasticizer migration.
 12. A method for manufacturing acompostable multilayer film, said method comprising the step of:coextruding at least three layers, said layers comprising (a) core layercomprising a polylactide polymer, the core layer having a T_(g) belowabout 20° C., and having first and second opposed surfaces; (b) firstblocking reducing layer covering the first surface of said core layer;and (c) second blocking reducing layer covering the second surface ofsaid core layer; wherein at least one of said blocking reducing layerscomprises a semicrystalline polymer composition comprising an aliphaticpolyester polymer.
 13. The method for manufacturing a compostablemultilayer film according to claim 12, further comprising a step ofblowing the layers.
 14. The method for manufacturing a multilayer filmaccording to claim 13, wherein the step of blowing comprises doublebubble blowing.
 15. The method for manufacturing a multilayer filmaccording to claim 12, further comprising the step of blow molding. 16.A compostable multilayer structure comprising: (a) compostablesubstrate; (b) core layer comprising a polylactide polymer compositioncovering said compostable substrate, said core layer having a T_(g)below about 20° C.; and (c) blocking reducing layer covering said corelayer; wherein said blocking reducing layer comprises a semicrystallinepolymer composition comprising an aliphatic polyester polymer.
 17. Thecompostable multilayer structure according to claim 16, wherein saidcompostable substrate comprises a cellulose-containing substrate. 18.The compostable multilayer structure according to claim 16, wherein thecellulose-containing substrate is paper.
 19. The compostable multilayerstructure according to claim 16, wherein said core layer comprises apolylactide polymer and an effective amount of a plasticizer to providea T_(g) for the core layer of below about 20° C.
 20. A compostablemultilayer film comprising: (a) core layer comprising a polylactidepolymer and at least 20% by weight, based on the weight of the corelayer, of a plasticizer, said core layer having first and second opposedsurfaces; (b) first blocking reducing layer covering the first surfaceof said core layer; and (c) second blocking reducing layer covering thesecond surface of said core layer; wherein at least one of the blockingreducing layers comprises a polymer selected from the group consistingof polyethylene(oxalate), polyethylene(succinate),polybutylene(oxalate), polybutylene(succinate),polypentamethyl(succinate), polyhexamethyl(succinate),polyheptamethyl(succinate), polyoctamethyl(succinate),polyethylene(succinate-co-adipate), polybutylene(succinate-co-adipate),polybutylene(oxylate-co-succinate), polybutylene(oxylate-co-adipate),and mixtures thereof.
 21. A compostable multilayer film comprising: (a)core layer comprising a polylactide polymer composition exhibitingbonding between polylactide polymer chains, the core layer having aT_(g) below about 20° C., and having first and second opposed surfaces;(b) first blocking reducing layer covering the first surface of saidcore layer; and (c) second blocking reducing layer covering the secondsurface of said core layer; wherein at least one of said blockingreducing layers comprises a semicrystalline polymer composition.
 22. Acompostable multilayer film according to claim 21, wherein thepolylactide polymer composition exhibiting bonding between polylactidepolymer chains comprises peroxide modified polylactide polymer.
 23. Acompostable multilayer structure comprising: (a) compostable substrate;(b) core layer comprising a polylactide polymer composition exhibitingbonding between polylactide polymer chains, the core layer covering saidcompostable substrate and having a T_(g) below about 20° C.; and (c)blocking reducing layer covering said core layer and comprising asemicrystalline polymer composition.
 24. A compostable multilayer filmaccording to claim 23, wherein the polylactide polymer compositionexhibiting bonding between polylactide polymer chains comprises peroxidemodified polylactide polymer.